Cutter accuracy

[Neale]

Been trying out my new CNC router with a few simple jobs. Trying to cut slots in thin ply, I found that my cutter seemed to cut consistently under size. 3mm single-flute cutter gave 2.6mm slot, or equivalent result cutting opposite sides of plain block (I.e. block was oversize after cutting by same amount). Tried a couple of other single-flute cutters (4mm and 6mm) which also seem to cut about 0.4mm undersize. Difficult to directly measure diameter of these cutters, but putting the 6mm cutter along a straightedge, the tip does sit slightly below the line of the shank. Is this normal? Do I need to, in effect, calibrate the diameter of a cutter before using? Are 2-flute cutters more accurate?